I. Field of the Invention
The present invention relates to an oxygen sensor with a heater and, more particularly, to a compact oxygen sensor part of which has a heater thereon.
II. Description of the Prior Art
Some sensors among various types of sensors are kept above a predetermined temperature in use. Typical examples are an oxygen concentration cell type oxygen sensor with an oxygen ionic conductor (e.g., ZrO.sub.2) and an oxygen sensor which utilizes a resistance change in an oxide semiconductor (e.g., TiO.sub.2 or CoO). These sensors can only be used above a temperature of about 400 [.degree.C.] due to the following reasons. When a reaction sufficiently progresses in an exhaust gas, that is, when a chemical equilibrium state is established, the oxygen partial pressure in the theoretical air-fuel ratio is abruptly changed. If the oxygen concentration cell type oxygen sensor is used, the electromotive force abruptly changes. If the oxide semiconductor type oxygen sensor is used, the resistance abruptly changes. However, in practice, a small amount of combustible components (H.sub.2, CO, HC) coexists with oxygen, so that the reaction described above does not progress completely. In this condition, if a signal proportional to the oxygen partial pressure in the exhaust gas is simply produced by the sensor, the electromotive force or resistance which is based on the theoretical air-fuel ratio will not be abruptly changed. In order to eliminate the above drawback, a catalyst is used to accelerate the reaction inside the sensor or in the vicinity of the surface thereof so as to greatly change the electromotive force or resistance according to the theoretical air-fuel ratio. Therefore, the lower operating limit of the catalyst determines that of the oxygen sensor. Since the reaction acceleration effect of the catalyst cannot be obtained at temperatures of not lower than about 400 [.degree.C.], the conventional oxygen sensor can only be used at temperatures of not lower than 400 [.degree.C.].
Alternatively, a limiting current type oxygen sensor has been proposed which has an anode and a cathode, and a means disposed at the cathode for limiting the inflow of a gas to be measured. When a predetermined DC voltage is applied across the oxygen sensor of this type, the sensor measures a limiting current flowing therethrough to detect the oxygen concentration. However, since the internal resistance is increased at a relatively low temperature, an output proportional to the oxygen concentration cannot be obtained. Therefore, the limiting current type oxygen sensor is not suitable for measurement at low temperatures.
In order to eliminate the drawback described above, a heater must be arranged in the limiting current type oxygen sensor so as to measure the oxygen concentration at low temperatures. However, it is desirable that the heating temperature be as high as 800 [.degree.C.] to 900 [.degree.C.] and that power consumption be as low as about 10 [W].
Sensors are often mounted in a small space. The typical example is an oxygen sensor which measures the distribution of the air-fuel ratio in an exhaust pipe of a vehicle. A sensor of this type must be very small, and so a heater used therein must also be small in size and light in weight. Even when there is no space-limitation factor, a compact and light-weight sensor and heater are desirable.
Further, sensors are often used in severe circumstances. For example, they may be used in a gas or liquid, or in an atmosphere where temperature change is great. Heaters for heating sensors of this type must have excellent durability and must not substantially degrade over time in the severe circumstances.
Further, a change in output of a sensor occurs with a change in the ambient temperature; the sensor output depends upon temperature variation. In order to eliminate temperature dependency of the sensor, the heating power of the heater is controlled according to the sensor temperature so as to keep its temperature constant. In order to keep the sensor temperature constant even if the ambient temperature is abruptly changed, there must be substantially no time lag between the change in heating power of the heater and the resulting change in the sensor temperature; excellent response time between the variables is required.
It is also desirable that sensor heaters be capable of being mass-produced at low cost with excellent mechanical strength, and that temperature control be easy.
Conventional heating methods for heating sensors have been proposed as follows, but none of these sufficiently satisfies the above needs.
A first heating method is performed by arranging a separate heater in the vicinity of the sensor to heat the sensor.
In a clean atmospheric environment, a tungsten wire, Kanthal alloy wire or the like is simply used as the heater. In severe circumstances such as a gaseous atmosphere or an exhaust gas atmosphere, or when immersed in water, a sheath wire is used wherein a tungsten, Kanthal alloy, or nichrome wire is coated with MgO powder and is embedded in a stainless steel or Inconel pipe. However, the heater itself becomes larger in size and the power consumption of the heating is more than several tens of watts. Therefore, the heating method described above is not suitable for the sensor under discussion.
A second heating method is performed by forming a sensor integrally with a heater for efficient heating. For this purpose, there are some conventional techniques whereby, for example, the heater is embedded in the sensor, or, alternatively, a heater pattern is printed on the lower surface of a substrate by a screen printing technique. However, the first technique does not provide a compact heater, and the heater wire according to the second technique is open to the atmosphere, so that it is greatly degraded over time (especially, in a "rich" mixture of the air-fuel ratio). Further, in the currently adopted printing method, the heater can hardly be formed with dimensions smaller than 150 [.mu.m] width and 10 [.mu.m] thickness. Micropatterning of the heater results in a size from ten up to about twenty millimeters. Thus, the heater pattern cannot be effectively used for the sensor under discussion. Neither the first nor the second heating method can sufficiently satisfy the various characteristics described above.